Lithographic apparatus and device manufacturing method

ABSTRACT

A lithographic apparatus includes a substrate table constructed to hold a substrate, a projection system configured to project a patterned radiation beam through an opening and onto a target portion of the substrate, and a conduit having an outlet in the opening. The conduit is configured to deliver gas to the opening. The lithographic apparatus includes a temperature control apparatus disposed in a space between the projection system and the substrate table. The temperature control device is configured to control the temperature of the gas in the space after the gas passes through the opening.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/356,358, filed May 5, 2014, now allowed, which is the national stageof PCT Patent Application No. PCT/EP2012/070247, filed Oct. 12, 2012,which claims priority to U.S. Provisional Application No. 61/561,117,filed on Nov. 17, 2011, the contents of which applications areincorporated by reference herein in their entireties.

FIELD

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned.

Lithography is widely recognized as one of the key steps in themanufacture of ICs and other devices and/or structures. However, as thedimensions of features made using lithography become smaller,lithography is becoming a more critical factor for enabling miniature ICor other devices and/or structures to be manufactured.

A theoretical estimate of the limits of pattern printing can be given bythe Rayleigh criterion for resolution as shown in equation (1):

$\begin{matrix}{{CD} = {k_{1}*\frac{\lambda}{NA}}} & (1)\end{matrix}$

where A is the wavelength of the radiation used, NA is the numericalaperture of the projection system used to print the pattern, k₁ is aprocess dependent adjustment factor, also called the Rayleigh constant,and CD is the feature size (or critical dimension) of the printedfeature. It follows from equation (1) that reduction of the minimumprintable size of features can be obtained in three ways: by shorteningthe exposure wavelength A, by increasing the numerical aperture NA or bydecreasing the value of k₁.

In order to shorten the exposure wavelength and, thus, reduce theminimum printable size, it has been proposed to use an extremeultraviolet (EUV) radiation source. EUV radiation is electromagneticradiation having a wavelength within the range of 5-20 nm, for examplewithin the range of 13-14 nm, or example within the range of 5-10 nmsuch as 6.7 nm or 6.8 nm. Possible sources include, for example,laser-produced plasma sources, discharge plasma sources, or sourcesbased on synchrotron radiation provided by an electron storage ring.

EUV radiation may be produced using a plasma. A radiation system forproducing EUV radiation may include a laser for exciting a fuel toprovide the plasma, and a source collector module for containing theplasma. The plasma may be created, for example, by directing a laserbeam at a fuel, such as particles of a suitable material (e.g. tin), ora stream of a suitable gas or vapor, such as Xe gas or Li vapor. Theresulting plasma emits output radiation, e.g., EUV radiation, which iscollected using a radiation collector. The radiation collector may be amirrored normal incidence radiation collector, which receives theradiation and focuses the radiation into a beam. The source collectormodule may include an enclosing structure or chamber arranged to providea vacuum environment to support the plasma. Such a radiation system istypically termed a laser produced plasma (LPP) source.

In order to project a pattern onto a substrate with a desired accuracyusing an EUV lithographic apparatus, it is desirable to control thetemperature of the substrate. This is because an uncontrolled change ofthe substrate temperature may cause the substrate to expand or contractsuch that the projected pattern is not positioned with a desiredaccuracy on the substrate (e.g. is not overlaid with a desired accuracyon a pattern already present on the substrate). A known problem in thisregard is that a purge gas that is used to prevent contaminants,originating from the exposed resist and/or wafer stage compartment,causes a net positive heat load on the wafer surface. This heat load, inturn, can result in large undesirable thermal deformations of the wafer.

SUMMARY

According to an aspect of the invention, there is provided alithographic apparatus comprising a substrate table constructed to holda substrate, a projection system configured to project a patternedradiation beam through an opening and onto a target portion of thesubstrate, and a conduit configured to deliver gas to the opening, andto provide a flow of the gas out of the opening to a space between theprojection system and the substrate table, wherein the lithographicapparatus is provided with a temperature control device configured tocontrol the temperature of the gas in said space after the gas passesthrough the opening.

In an embodiment of the invention, the temperature control devicecomprises temperature control means, or a temperature controller, whichincludes both heating means, or a heater, and cooling means, or acooler. The heating and cooling means are configured to provide a netcooling effect to the gas.

Desirably, the heating means and the cooling means are disposed in aspace between the projection system and the substrate table. The heatingmeans and the cooling means may be mounted on a support member locatedin the space. The support member may be mounted on a surface of theprojection system facing the substrate table and either in thermalcommunication with said surface or thermally insulated from saidsurface.

The heating means may comprise at least one resistive heating element.Desirably, the at least one resistive heating element comprises aplurality of independently controllable segments. In other embodimentsthere may be a plurality of independently controllable resistive heatingelements.

Desirably, the cooling means comprises at least one cooling elementwhich may desirably be at least one heat-pipe arranged to carry acooling fluid cooled at a remote source.

In embodiments of the invention, the temperature control means isconfigured to control the amount of cooling by controlling the heatingmeans.

Some embodiments of the invention may further comprise a metrologyapparatus configured to measure overlay of a pattern projected onto thesubstrate by the lithographic apparatus, wherein a control system isconfigured to determine an adjustment of operation of the temperaturecontrol device based upon an output from the metrology apparatus. Thelithographic apparatus may further comprise a measurement apparatusconfigured to measure infrared reflectivity of the substrate. Thecontrol system may be configured to determine an adjustment of operationof the temperature control device based on the measured reflectivity ofthe substrate.

According to another aspect of the invention there is provided a devicemanufacturing method comprising projecting a patterned beam of radiationthrough an opening in a projection system onto a substrate, anddelivering gas to the opening in the projection system via a conduit,wherein the method further comprises controlling the temperature of thegas such that the temperature of the gas is controlled in a spacebetween the projection system and the substrate after passing throughthe opening.

Desirably, the temperature control comprises cooling that is achieved byover-cooling using cooling means and heating using heating means.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 schematically depicts a lithographic apparatus according to anembodiment of the invention;

FIG. 2 schematically depicts a more detailed view of the apparatus ofFIG. 1, including a discharge produced plasma source collector module;

FIG. 3 schematically depicts a view of an alternative source collectormodule of the apparatus of FIG. 1, the alternative being a laserproduced plasma source collector module;

FIG. 4 schematically depicts part of a projection system of thelithographic apparatus and a substrate as held by a substrate table;

FIG. 5 is a detailed view of part of FIG. 4 showing the heating andcooling means.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus 100 according toan embodiment of the invention. The apparatus comprises: an illuminationsystem (illuminator) IL configured to condition a radiation beam B (e.g.EUV radiation); a support structure (e.g. a mask table) MT constructedto support a patterning device (e.g. a mask or a reticle) MA andconnected to a first positioner PM configured to accurately position thepatterning device; a substrate table (e.g. a wafer table) WT constructedto hold a substrate (e.g. a resist-coated wafer) W and connected to asecond positioner PW configured to accurately position the substrate;and a projection system (e.g. a reflective projection system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure MT holds the patterning device MA in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem.

The term “patterning device” should be broadly interpreted as referringto any device that can be used to impart a radiation beam with a patternin its cross-section such as to create a pattern in a target portion ofthe substrate. The pattern imparted to the radiation beam may correspondto a particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The projection system, like the illumination system, may include varioustypes of optical components, such as refractive, reflective, magnetic,electromagnetic, electrostatic or other types of optical components, orany combination thereof, as appropriate for the exposure radiation beingused, or for other factors such as the use of a vacuum. It may bedesired to use a vacuum for EUV radiation since gases may absorb toomuch radiation. A vacuum environment may therefore be provided to thewhole beam path with the aid of a vacuum wall and vacuum pumps. Some gasmay be provided in some parts of the lithographic apparatus, for exampleto allow gas flow to be used to reduce the likelihood of contaminationreaching optical components of the lithographic apparatus.

As here depicted, the apparatus is of a reflective type (e.g. employinga reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives an extreme ultra violet(EUV) radiation beam from the source collector module SO. Methods toproduce EUV light include, but are not necessarily limited to,converting a material into a plasma state that has at least one element,e.g., xenon, lithium or tin, with one or more emission lines in the EUVrange. In one such method, often termed laser produced plasma (“LPP”),the desired plasma can be produced by irradiating a fuel, such as adroplet, stream or cluster of material having the required line-emittingelement, with a laser beam. The source collector module SO may be partof an EUV radiation system including a laser, not shown in FIG. 1, forproviding the laser beam exciting the fuel. The resulting plasma emitsoutput radiation, e.g. EUV radiation, which is collected using aradiation collector, disposed in the source collector module. The laserand the source collector module may be separate entities, for examplewhen a CO₂ laser is used to provide the laser beam for fuel excitation.

In such cases, the laser is not considered to form part of thelithographic apparatus and the radiation beam is passed from the laserto the source collector module with the aid of a beam delivery systemcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases, the source may be an integral part of thesource collector module, for example when the source is a dischargeproduced plasma EUV generator, often termed as a DPP source.

The illuminator IL may comprise an adjuster for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as facetted field and pupilmirror devices. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g. mask)MA, which is held on the support structure (e.g. mask table) MT, and ispatterned by the patterning device. After being reflected from thepatterning device (e.g. mask) MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW andposition sensor PS2 (e.g. an interferometric device, linear encoder orcapacitive sensor), the substrate table WT can be moved accurately, e.g.so as to position different target portions C in the path of theradiation beam B. Similarly, the first positioner PM and anotherposition sensor PS1 can be used to accurately position the patterningdevice (e.g. mask) MA with respect to the path of the radiation beam B.Patterning device (e.g. mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.

The depicted apparatus may be used in at least one of the followingmodes:

-   1. In step mode, the support structure (e.g. mask table) MT and the    substrate table WT are kept essentially stationary, while an entire    pattern imparted to the radiation beam is projected onto a target    portion C at one time (i.e. a single static exposure). The substrate    table WT is then shifted in the X and/or Y direction so that a    different target portion C can be exposed.-   2. In scan mode, the support structure (e.g. mask table) MT and the    substrate table WT are scanned synchronously while a pattern    imparted to the radiation beam is projected onto a target portion C    (i.e. a single dynamic exposure). The velocity and direction of the    substrate table WT relative to the support structure (e.g. mask    table) MT may be determined by the (de-)magnification and image    reversal characteristics of the projection system PS.-   3. In another mode, the support structure (e.g. mask table) MT is    kept essentially stationary holding a programmable patterning    device, and the substrate table WT is moved or scanned while a    pattern imparted to the radiation beam is projected onto a target    portion C. In this mode, generally a pulsed radiation source is    employed and the programmable patterning device is updated as    required after each movement of the substrate table WT or in between    successive radiation pulses during a scan. This mode of operation    can be readily applied to maskless lithography that utilizes    programmable patterning device, such as a programmable mirror array    of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 shows the apparatus 100 in more detail, including the sourcecollector module SO, the illumination system IL, and the projectionsystem PS. The source collector module SO is constructed and arrangedsuch that a vacuum environment can be maintained in an enclosingstructure 220 of the source collector module SO. An EUV radiationemitting plasma 210 may be formed by a discharge produced plasma source.EUV radiation may be produced by a gas or vapor, for example Xe gas, Livapor or Sn vapor in which the very hot plasma 210 is created to emitradiation in the EUV range of the electromagnetic spectrum. The very hotplasma 210 is created by, for example, an electrical discharge causingan at least partially ionized plasma. Partial pressures of, for example,10 Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may berequired for efficient generation of the radiation. In an embodiment, aplasma of excited tin (Sn) is provided to produce EUV radiation.

The radiation emitted by the hot plasma 210 is passed from a sourcechamber 211 into a collector chamber 212 via an optional gas barrier orcontaminant trap 230 (in some cases also referred to as contaminantbarrier or foil trap) which is positioned in or behind an opening insource chamber 211. The contaminant trap 230 may include a channelstructure. Contamination trap 230 may also include a gas barrier or acombination of a gas barrier and a channel structure. The contaminanttrap or contaminant barrier 230 further indicated herein at leastincludes a channel structure, as known in the art.

The collector chamber 212 may include a radiation collector CO which maybe a so-called grazing incidence collector. Radiation collector CO hasan upstream radiation collector side 251 and a downstream radiationcollector side 252. Radiation that traverses collector CO can bereflected off a grating spectral filter 240 to be focused in a virtualsource point IF. The virtual source point IF is commonly referred to asthe intermediate focus, and the source collector module is arranged suchthat the intermediate focus IF is located at or near an opening 221 inthe enclosing structure 220. The virtual source point IF is an image ofthe radiation emitting plasma 210.

Subsequently the radiation traverses the illumination system IL, whichmay include a facetted field mirror device 22 and a facetted pupilmirror device 24 arranged to provide a desired angular distribution ofthe radiation beam 21, at the patterning device MA, as well as a desireduniformity of radiation intensity at the patterning device MA. Uponreflection of the beam of radiation 21 at the patterning device MA, heldby the support structure MT, a patterned beam 26 is formed and thepatterned beam 26 is imaged by the projection system PS via reflectiveelements 28, 30 onto a substrate W held by the wafer stage or substratetable WT.

More elements than shown may generally be present in illumination opticsunit IL and projection system PS. The grating spectral filter 240 mayoptionally be present, depending upon the type of lithographicapparatus. Further, there may be more mirrors present than those shownin the figures, for example there may be 1-6 additional reflectiveelements present in the projection system PS than shown in FIG. 2.

Collector optic CO, as illustrated in FIG. 2, is depicted as a nestedcollector with grazing incidence reflectors 253, 254 and 255, just as anexample of a collector (or collector mirror). The grazing incidencereflectors 253, 254 and 255 are disposed axially symmetric around anoptical axis O and a collector optic CO of this type is desirably usedin combination with a discharge produced plasma source, often called aDPP source.

Alternatively, the source collector module SO may be part of a laserproduced plasma (LPP) radiation system, as shown in FIG. 3. A laser LAis arranged to deposit laser energy into a fuel, such as xenon (Xe), tin(Sn) or lithium (Li), creating the highly ionized plasma 210 withelectron temperatures of several 10's of eV. The energetic radiationgenerated during de-excitation and recombination of these ions isemitted from the plasma, collected by a near normal incidence collectoroptic CO and focused onto the opening 221 in the enclosing structure220.

FIG. 4 shows schematically in cross-section a lower portion of theprojection system PS, i.e., a portion facing the substrate holder WT.The projection system PS includes an opening 301 through which in use apatterned radiation beam is projected onto a target portion of thesubstrate. The opening may be formed by a sloped inner surface 304 of anopening defining wall 302. Wall 302 is hollow and defines an annularchamber 305 which receives gas from a gas supply (e.g. via a deliverypipe not shown) and delivers gas to the opening 301 through an annularslit 306 formed in the sloped inner surface 304. The delivery pipe,chamber 305 and slit 306 may together be considered to form a conduitwhich delivers gas to the opening 301. The conduit may have any othersuitable form that serves to supply gas to the opening 301.

The annular slit 306 is an example of an outlet. The outlet may have anysuitable form. The outlet may for example comprise a plurality of holes.The plurality of holes may be distributed around the outlet. The holesmay be rectangular, square, circular, or may have any other suitableshape. The reference 306 in FIG. 4 may refer to any such outlets; anoutlet will also be referred to as outlet 306 hereinafter.

It is desirable to reduce the likelihood that contamination (e.g.gas-phase organic compounds coming from the resist on the substrate W)will travel from the substrate W into the interior of the projectionsystem PS. This is because the contamination may accumulate on opticalsurfaces such as the mirrors 28, 30 and cause the reflectivity of suchsurfaces to be reduced. This in turn may reduce the intensity of EUVradiation available for projection onto the substrate W, and thereforereduce the throughput of the lithographic apparatus (i.e. the number ofsubstrates which may be patterned per hour by the lithographicapparatus).

The gas supplied to the opening 301 through the annular slit 306 servesas a purge gas that reduces the likelihood of contamination passing fromsubstrate W through the opening 301 and into the projection system PS.The flow of gas is schematically represented by arrows in FIG. 4. Thegas travels into and within the annular chamber 305 such that thepressure of gas at the entrance to the annular slit 306 is substantiallyequal around the circumference of the annular chamber 305. There may besome variation in pressure around the annular chamber 305 and to correctfor this the chamber may include a baffle (not shown) which acts toencourage the gas to travel within the annular chamber, therebyassisting with equalization of the pressure of the gas within theannular chamber. Gas passes through the annular slit 306 and into theopening 301. A portion of the gas travels upwards into the projectionsystem PS. The remainder of the gas travels downwards, passing out ofthe opening 301 and then traveling away from the opening in a gap orspace between the projection system PS and the substrate W. The flow ofgas out of the opening 301 prevents or suppresses the passage ofcontamination from the substrate W into the projection system PS.

It is desirable to control the temperature of the substrate W. This isbecause an uncontrolled change of the substrate temperature may causethe substrate to expand or contract such that a projected pattern is notpositioned with a desired accuracy on the substrate (e.g. is notoverlaid with a desired accuracy on a pattern already present on thesubstrate). The flow of gas from the opening 301 onto the substrate Wmay heat the substrate in an unwanted manner that will be describedfurther below. It may be desirable to prevent the flow of gas fromheating the substrate in an unwanted manner. Additionally at least partsof the substrate may experience higher temperatures due to heatingcaused by EUV and infrared radiation incident on parts of the substrate.The infrared radiation may for example arise from an EUV emitting plasma210 or a laser LA used in the generation of the EUV emitting plasma (seeFIG. 3).

In an embodiment, the gas is cooled as it flows from the opening 301 tothe edge of the substrate W. To provide the cooling of the gas as itflows from the opening 301 to the edge of the substrate W, thelithographic apparatus comprises a temperature control device 330described in detail below. The temperature control device 330 isconfigured to control the temperature of the gas in the space betweenthe projection system PS and the substrate W, after passing through theopening 301. The temperature of the gas may be controlled such that thetemperature of the substrate W is not modified by the gas to such anextent that it causes unacceptable overlay errors to occur in thelithographic apparatus. It is appreciated that the space between theprojection system PS and the substrate W is equivalent to the spacebetween the projection system and the substrate table WT for the purposeof describing the technical features of the embodiment when theapparatus is not in use. In view of this equivalence there will not bemade a distinction between the two definitions of this space in thedescription of the invention hereinafter.

The lithographic apparatus may have a reference temperature, which mayfor example be around 295 Kelvin (e.g. around room temperature which maybe between 20 and 23° C.) or may be some other temperature. Thereference temperature may be the temperature at which one or morecomponents of the lithographic apparatus are held during operation ofthe lithographic apparatus. The lithographic apparatus may hold (orendeavour to hold) one or more of the substrate W, substrate table WT,projection system walls and opening defining wall 302 at the referencetemperature. If the lithographic apparatus were to operate without theflow of gas, then the substrate W, substrate table WT, projection systemwalls and opening defining wall 302 may be at the reference temperature(or substantially at the reference temperature). An exception to thismay be a portion of the substrate which is being exposed; this may havea higher temperature due to heating caused by EUV and infrared radiationincident on that portion of the substrate. The infrared radiation mayfor example arise from an EUV emitting plasma 210 or a laser LA used inthe generation of the EUV emitting plasma (see FIG. 3).

When gas is flowing through the apparatus it may cause heating of thesubstrate W, even if the gas which is introduced into the chamber 305initially has the same temperature as other components of thelithographic apparatus. This is because gas acquires heat as it travelsto the substrate W, the heat then being transferred to the substratefrom the gas. This heating of the substrate may lead to unacceptableoverlay errors occurring during exposure of the substrate.

The mechanism via which the gas acquires heat as it travels to thesubstrate W is a consequence of the velocity at which the gas travels.Gas which is travelling to the annular chamber 305 through a deliverypipe will have a relatively high velocity (e.g. 100 m/s or greater).This relatively high velocity of the gas will cause a reduction of thetemperature of the gas as it is flowing to the chamber. This may beunderstood by considering an example in which the gas is adiabatic (i.e.with no heat transfer taking place between the delivery pipe and thegas). If the gas is adiabatic then the total energy of the gas mustremain constant. If the gas is stationary or slow moving (e.g. 10 m/s orless) when it enters the delivery pipe then all of the energy of the gasis thermal energy, and this is manifest as the temperature of the gas.However, when the gas is travelling at a relatively high velocity in thedelivery pipe, the gas has significant kinetic energy due to itsvelocity. Since the total energy of the gas remains the same, thethermal energy of the gas (and hence its temperature) is reduced. Thus,in the adiabatic case, when a gas flows at a relatively high velocity,the temperature of the gas will be reduced.

The gas in an embodiment of the invention may flow at a relatively highvelocity through the delivery pipe, and as a result of this relativelyhigh velocity, the static temperature of the gas is reduced. If the gasand the projection system wall both have the same initial temperature(the initial gas temperature being the temperature of the gas before itflows through the delivery pipe), then when the gas is flowing throughthe delivery pipe it will have a static temperature which is lower thanthe temperature of the projection system wall. Because the gas has alower static temperature than the projection system wall, heat will flowfrom the projection system wall to the gas as it travels through thedelivery pipe. In an alternative embodiment, the gas may flow throughthe delivery pipe at a relatively low velocity, in which case thetemperature of the gas will not be reduced. However, the temperature ofthe gas will be reduced when the gas flows through the annular slit 306.

The annular slit 306 through which the gas passes into the opening 301is relatively constricted. Consequently, the gas will travel through theannular slit 306 at a relatively high velocity, and this will cause thetemperature of the gas to be reduced (e.g. by several Kelvin or even upto tens of Kelvins). If the gas has a lower temperature than the openingdefining wall 304 as it travels through the annular slit 306, heat willbe transferred from the opening defining wall to the gas.

The gas may have a relatively high velocity when it enters the opening301. However, the gas decelerates when it is incident upon the surfaceof the substrate W, since the substrate prevents the gas from continuingto travel downwards and forces the gas to change direction. As a resultof this deceleration, kinetic energy in the gas is converted to thermalenergy. Since heat has been transferred to the gas from the deliverypipe and the opening defining wall 304, the gas has a total temperaturewhich is higher than its total temperature before it entered thedelivery pipe. The total temperature of the gas may for example behigher than the reference temperature of the lithographic apparatus. Itis this higher total temperature of the gas which may cause undesirableheating of the substrate W.

In an embodiment, the opening defining wall 302 may be insulated toreduce the amount of heat which may flow to the opening defining wallfrom the projection system walls (or other parts of the lithographicapparatus). This reduces the amount of heat that may be transferred tothe gas as it passes through the annular slit 306. The insulation mayfor example comprise providing a gap and/or insulating material betweenthe opening defining wall 302 and the projection system walls (thelatter walls not shown in FIG. 4).

In an embodiment, the opening defining wall 302 may be constructed froman insulating material. For example, the opening defining wall 302 maybe constructed from a ceramic, e.g. Macor ceramic, which is availablefrom Corning Inc. of Corning, USA (or some other suitable ceramic). Theopening defining wall 302 may be formed from glass. The opening definingwall 302 may be formed from a metal which has a lower thermalconductivity than some other metals. For example, the opening definingwall 302 may be formed from stainless steel, which may providestructural strength and which has a thermal conductivity significantlylower than that of aluminium.

In the embodiment of FIG. 4, the chamber 305 has an annular lower wall303 that is parallel to the substrate W and which together with thesubstrate W defines a gap through which the gas flows. Secured to thelower wall 303 is a support member 320 in the form of an annular diskthat carries temperature control means (or a temperature controller) tobe described below, and that together with the temperature control meansis part of aforementioned temperature control device 330. Support member320 is provided with an opening 321 that is contiguous with the opening301 so as not to impede the flow of gas or block the exposure radiationbeam. Support member 320 serves to support temperature control means aswill be described below and while an annular disk is a particularconvenient form for the support member it may take any other suitableform.

FIG. 5 shows in more detail a section of the support member 320extending from the edge of opening 321 to the outer circumference of thesupport member 320. Supported by the support member 320 are at least oneheating means 322 (or heater) and at least one cooling means 323 (orcooler). Cooling means 323 may take any suitable form e.g. with acircular or rectangular cross section but desirably comprises a tubecarrying cooling fluid supplied from a remote cooling unit (e.g. aPeltier cooler connected via a heat-pipe). Cooling can be delivered bymeans of, for example but not limited to, use of a heat pipe or asingle-phase cooling fluid such as water or glycol. Heating means 322may comprise at least one resistive heating element. The support member320 is desirably formed of a thermally conducting material and isdesirably a material with low outgassing so as not to risk addingcontaminants to the substrate W beneath the support member 320. Suitablematerials include aluminum, steel, and ceramics such as aluminum nitrideor silicon carbide.

It should be noted that in FIG. 5, the heating and cooling means areboth shown below the support member 320 whereas in FIG. 4 they are shownabove the support member 320. Both options are possible.

The support member 320 is desirably secured to the lower wall 303 ofchamber 305 though it may also be held in place by other means withoutnecessarily contacting wall 303. Optionally, in some embodiments of theinvention, the system may be arranged to pre-cool gas within the chamber305. This may be done by ensuring that the support member 320 is in goodthermal contact with the lower wall 303. Conversely, pre-cooling the gasin the chamber 305 may have some disadvantages in that the cooling mightundesirably cool neighboring sub-modules or parts thereof. In this case,the support member 320 may be mounted separately so that it is adjacentbut not in contact with the lower wall 303, or it may be fixed to thelower wall 303 with a thermally insulating material therebetween.

Although both heating and cooling means are provided, the primaryobjective is to provide a variable cooling effect on the gas as it flowsfrom the opening 301 to the edge of the substrate W. A potentialadvantage of providing both heating and cooling means is that theresponse time of the system can be improved. The response time ofcooling means is generally much slower than the response time of heatingmeans. Embodiments of the present invention provide cooling with a rapidresponse time by using the cooling means 323 to over-cool the gas (i.e.to provide a degree of cooling that on its own would cool the gas to alower temperature than is desired) and then to use the heating means 322to heat the gas so that there is a net cooling effect. By keeping theover-cooling substantially constant, it is the heating means 322 thatcontrols the net cooling and as the heating means has a relatively fastresponse time, the response time of the system is relatively fast.Optionally, only cooling or only heating may be provided, but the bestresults may be obtained with over-cooling combined with heating toproduce a variable cooling that can be accurately controlled.

As explained, the system may provide over-cooling of the gas incombination with heating. This provides a wide range of control of thecooling with a rapid response time. The over-cooling is desirably leftunchanged and control is provided by adjusting the heating. Control isenabled by providing a temperature feedback or feed-forward signal to acontrol system (not shown in any of the figures) constructed andarranged to control the operation of the temperature control device. Inparticular, the control system may be constructed and arranged to set oradjust respective temperatures of the heating means 322 and coolingmeans 323. This signal may be obtained from one or more temperaturesensor(s) located on or adjacent to the disk 320. In some embodiments ofthe invention, the resistive heating element(s) may themselves act asone or more temperature sensors by monitoring the resistivity of theheating element(s).

In embodiments of the invention control signals may also be providedfrom other influencing modules of the lithographic apparatus to thecontrol system. For example, signals indicative of the radiation energyfor wafer exposure and the wafer chuck temperature may be provided aspart of the feedback control.

The heating and cooling means 322, 323 may be configured on the disk 320in a wide range of different configurations. In a particularly simpleconfiguration there may be only a single heating element and a singlecooling element, but desirably to provide the maximum degree of control,there may be a plurality of independently controllable heating elementsor a single heating element sub-divided into independently controllablesegments. Providing independently controllable heating elements orsub-divided independently controllable segments may allow the degree ofnet cooling to be controlled over different parts of the support member.Normally, for example, there will be a greater degree of coolingadjacent to the opening 301 as the gas is at its warmest at that point.Similarly, there may be multiple cooling elements to provide the maximumdegree of control. In some embodiments, the heating and cooling means322, 323 on the support member 320 may be provided into a number ofdifferent independently controllable sectors, each of which include atleast one heating element or independently controllable heating elementsegment and at least one cooling element or part thereof.

In an embodiment, the temperature to which the temperature controldevice 330 cools the gas may be used (at least in part) to adjust theoverlay achieved by the lithographic apparatus. Overlay may beconsidered to be a measurement of the accuracy with which thelithographic apparatus projects a pattern on top of a pattern alreadypresent on the substrate. The overlay achieved by the lithographicapparatus may be measured following exposure of a substrate by using ametrology apparatus (which may form part of the lithographic apparatus)to measure the positions of projected patterns relative to patternspreviously present on the substrate. If a sub-optimal overlay is found,the pattern of which is indicative of the substrate having a temperaturewhich is too high, then the cooling provided by the temperature controldevice 330 may be increased. Conversely, if a sub-optimal overlay isfound, the pattern of which is indicative of the substrate having atemperature which is too low, then the cooling provided by thetemperature control device 330 may be reduced or the amount ofcompensation heating may be increased. The cooling provided by thetemperature control device 330 may be adjusted periodically in order tomaintain a desired overlay accuracy.

The absorption of EUV radiation by the substrate may be relativelyconstant and predictable. However, the absorption of IR radiation by thesubstrate may vary significantly depending upon the form of the surfaceof the substrate. For example, if a structure has previously beenexposed and processed on the substrate then the absorption of IRradiation by the substrate will depend upon the form of that structure.If the structure is formed from metal then the absorption of IRradiation will be less than the absorption would have been if thestructure was formed from semiconductor material. The adjustment of thetemperature of the gas delivered from the temperature control device 330may take into account the reflectivity to and absorptivity of infraredradiation of substrates being exposed (or to be exposed) by thelithographic apparatus. In an embodiment, the reflectivity to andabsorptivity of infrared radiation of a substrate (which may berepresentative of a plurality of substrates) may be performed in ameasurement apparatus prior to exposure of the substrate (or pluralityof substrates) by the lithographic apparatus. The measurement apparatusmay form part of the lithographic apparatus. For example, an apparatusmay direct an infrared radiation beam at the substrate and detectradiation reflected from the substrate, thereby allowing thereflectivity of the substrate to the infrared radiation to bedetermined. The absorptivity may also be measured. The infraredradiation may for example be 10.6 μm, and may for example be provided bya laser. The apparatus may for example be an apparatus that is used toperform measurements of other properties of the substrate (such anapparatus may be referred to as a metrology apparatus). The controlsystem may adjust the temperature of the heating means 322, 333 to takeinto account the IR reflectivity and absorptivity of substrates whichare to be exposed (or are being exposed) by the lithographic apparatus(for example, if the gas is being used to provide cooling that at leastpartially compensates for heating of the substrate caused by IRradiation).

The heating means 322 and cooling means 323 may be disposed on thesupport member 320 with rotational symmetry. This is particularly usefulwhere the gas flows radially outwardly from the opening 301 in alldirections equally. However, either by means of independent control ofheating elements or segments of heating elements, or by locating theheating and cooling elements in particular chosen configurations,specific patterns of cooling can be obtained that may be suitable inspecific applications where for any reason there is a need to providesuch a specific cooling pattern.

Embodiments of the present invention allow for the possibility thatthere may be a local relative raised wafer temperature at positions justfacing the outlet 301. This may be acceptable because such a centralarea of locally higher temperature can be compensated for by an annulararea of lower temperature arranged around the central area, therebyexploiting the relatively high thermal conductance of a typical siliconwafer.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled person will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The term “EUV radiation” may be considered to encompass electromagneticradiation having a wavelength within the range of 5-20 nm, for examplewithin the range of 13-14 nm, or example within the range of 5-10 nmsuch as 6.7 nm or 6.8 nm.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practised otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein. The descriptions above are intendedto be illustrative, not limiting. Thus it will be apparent to oneskilled in the art that modifications may be made to the invention asdescribed without departing from the scope of the claims set out below.

1. A lithographic apparatus comprising: a substrate table constructed tohold a substrate; a projection system configured to project a patternedradiation beam through an opening and onto a target portion of thesubstrate; a conduit configured to deliver gas to the opening, andprovide a flow of the gas out of the opening to a space between theprojection system and the substrate table; and a temperature controldevice disposed in the space between the projection system and thesubstrate table, said temperature control device configured to controlthe temperature of the gas in said space after the gas passes throughsaid opening; wherein the opening is formed by a sloped inner surface ofan opening defining wall.
 2. The lithographic apparatus as claimed inclaim 1, wherein the wall defines an annular chamber.
 3. Thelithographic apparatus as claimed in claim 2, wherein the annularchamber receives gas from a gas supply and delivers gas to the openingthrough an annular slit formed in the sloped inner surface.
 4. Thelithographic apparatus as claimed in claim 1, wherein the temperaturecontrol device comprises temperature controllers including both a heaterand a cooler.
 5. The lithographic apparatus as claimed in claim 4,wherein the heater and the cooler are mounted on a support memberlocated in said space.
 6. The lithographic apparatus as claimed in claim5, wherein the heater and cooler are disposed on the support member witha rotational symmetry.
 7. The lithographic apparatus as claimed in claim5, wherein the support member is mounted on a surface of the projectionsystem facing said substrate table.
 8. The lithographic apparatus asclaimed in claim 7, wherein the support member is mounted in thermalcommunication with said surface.
 9. The lithographic apparatus asclaimed in claim 7, wherein the support member is thermally isolatedfrom said surface.
 10. The lithographic apparatus as claimed in claim 4,wherein said heater comprises at least one resistive heating element.11. The lithographic apparatus as claimed in claim 10, wherein said atleast one resistive heating element comprises a plurality ofindependently controllable segments.
 12. The lithographic apparatus asclaimed in claim 4, wherein said heater comprises a plurality ofindependently controllable heating elements.
 13. The lithographicapparatus as claimed in claim 4, wherein said cooler comprises at leastone cooling element.
 14. The lithographic apparatus as claimed in claim13, wherein said at least one cooling element comprises a heat-pipearranged to carry a cooling fluid cooled at a remote source.
 15. Thelithographic apparatus as claimed in claim 4, wherein said temperaturecontroller is configured to control the amount of cooling by controllingthe heating means.
 16. The lithographic apparatus as claimed in claim 1,further comprising a metrology apparatus configured to measure overlayof a pattern projected onto the substrate by the lithographic apparatus,wherein an output from the metrology apparatus is used as a controlinput to a control system constructed and arranged to control operationof the temperature control device.
 17. The lithographic apparatus asclaimed in claim 1, further comprising a measurement apparatusconfigured to measure infrared reflectivity of the substrate, whereinthe measured reflectivity of the substrate is used as a control input toa control system constructed and arranged to control operation of thetemperature control device.
 18. A device manufacturing methodcomprising: projecting a patterned beam of radiation through an openingin a projection system onto a substrate; delivering gas to the openingin the projection system via a conduit; and controlling the temperatureof the gas after the gas passes through said outlet such that thetemperature of the gas is controlled in a space between the projectionsystem and the substrate; wherein the temperature control comprisescooling that is achieved by cooperation of over-cooling using coolingmeans and heating using heating means; the opening is formed by a slopedinner surface of an opening defining wall.